Nanocrystalline particles, i.e. particles with physical size of about 1-100 nm, possess important technological properties ranging from superior mechanical behavior to novel electronic and magnetic properties. Unfortunately, nanocrystalline particles, by virtue of their size and high surface area, are very reactive and interact with their surroundings quickly. For example, metal nanocrystals tend to oxidize rapidly when exposed to air. Therefore, it would be most useful if such nanocrystals could be protected from the environment and still retain their intrinsic properties.
The desirability of the encapsulation of nanoparticles of metals inside graphite shells has been recognized. For instance, magnetic materials (such as metallic iron, cobalt or cobalt-chromium alloy) encapsulated inside graphite shells can find applications as recording media, ferro fluids or magnetic tagging elements. These nanocapsules may be injected into biological systems for use as a drug or a tracing delivery and monitoring system. These materials may also find applications in electronic and opto-electronic industries by virtue of their small particle size, which would give rise to novel quantum phenomena. The encapsulated nanoparticles may be consolidated or dispersed in a matrix to form interpenetrated composites which will have applications in areas which require better mechanical properties or unique electronic and magnetic properties.
Generally the prior art synthesis methods utilize an arc between two graphite electrodes in which one electrode (the anode) is a mixture of graphite and the material to be encapsulated, and the other electrode (the cathode) is graphite. Such a process generally results in isolated instances of encapsulation with a low yield. Furthermore, the process also produces a lot of empty graphite shells, graphite flakes, amorphous debris and graphite nanotubes which are difficult to separate from those which encapsulate the material of interest.
Other methods suffer from surface contamination by impurities, which can greatly change the desired properties of a bulk sample. The inert gas condensation method is one of the cleanest ways to produce nanophase materials. Because the material is physically evaporated in the absence of any precursors, there are no contaminants left on the surface of the particles, which eliminates a common problem with chemical methods. However, this method results in very small bulk samples due to the low production rate, and the properties measured from such small samples may not represent those of bulk nanophase materials. An arc synthesis method circumvents or minimizes many of these problems.
Therefore an object of the subject invention is nanocrystal encapsulation in graphite shell, and a method of making such a product.
A further object of the subject invention is a method to separate encapsulated nanocrystals from other debris.
These and other objects are attained by the subject invention which uses a tungsten arc method to produce graphite-encapsulated nanocrystals. An arc chamber is filled with an inert or reducing gas. A tungsten rod is used as a nonconsumable cathode. The anode is made up of the material which is to be synthesized into nanocrystalline form, for example a metal or alloy. The anode material could be a block or shot of material of interest and is held in a graphite crucible. In the preferred embodiment, the material is separated from the crucible by a few layers of graphite foil to reduce heat loss.
During operation of the arc, it is advantageous to direct a jet of helium (or an inert gas) at the arc for rapid quenching of the metal vapor generated in the arc and reducing the average diameter of the resulting nanophase particles. The product is collected in a suitable collection system, or later scraped from the walls of the arc chamber.
After collection, the product is washed with a strong acid/solvent to dissolve the unwanted unencapsulated particles, but not attack the particles that are encapsulated in graphite shells. The undissolved residue is almost exclusively nanocrystalline particles encapsulated by graphite shells.